In recent years, organic EL (Electro Luminescence) displays have been gaining attention as thin, lightweight, and fast-responsive display devices. As a method of performing gradation display with an organic EL display, there is known analog gradation driving for controlling a driving TFT (Thin Film Transistor) within a pixel circuit using an analog signal, and digital gradation driving for controlling the driving TFT using a digital signal. Compared to the analog gradation driving, the digital gradation driving provides higher tone reproduction and superior image quality.
In the following description, time-division gradation driving as one type of the digital gradation driving is taken as an example. The time-division gradation driving is a driving method in which one frame period is divided into a plurality of sub-frame periods, and a state of a display element is controlled to be in a light-emitting state or in a non-light-emitting state in each sub-frame period. Brightness of the display element in one frame period is determined based on a total length of sub-frame periods in which the display element is in the light-emitting state. The time-division gradation driving is used for driving of PDPs (Plasma Display Panels) as well.
There have been proposed various pixel circuits for the organic EL display (in the conventional pixel circuits described below, names of the components and the signal lines are changed in order to simplify comparison with the present invention). As shown in FIG. 6, Patent Document 1 describes a pixel circuit 60 including TFTs 61 to 63, a capacitor 64, and an organic EL element 65. As shown in FIG. 7, a potential of a control line Ei changes with a delay of a predetermined time from a potential of a control line Wi. When the potential of the control line Ei becomes a high level, the TFT 63 is turned to an ON state, the TFT 61 is turned to an OFF state, and the organic EL element 65 is turned to a non-light-emitting state. Therefore, it is possible to adjust display brightness of the organic EL element 65 by adjusting the length of delay time shown in FIG. 7.
As shown in FIG. 8, Patent Document 2 describes a pixel circuit 70 including TFTs 71 to 73, a capacitor 74, and an organic EL element 75. When a potential of a control line Ei becomes a high level, the TFT 73 is turned to an ON state, the TFT 71 is turned to an OFF state, and the organic EL element 75 is turned to a non-light-emitting state. The TFT 73 is provided for an organic EL display performing time-division gradation driving, in order to carry out writing and erasing of data in parallel.
As shown in FIG. 9, Patent Document 3 describes a pixel circuit 80 including TFTs 81 to 83, a capacitor 84, and an organic EL element 85. In the pixel circuit 80, a gate terminal and a drain terminal of the TFT 83 are connected to a control line Ei. When a potential of the control line Ei becomes a high level, a current flows from the control line Ei to a gate terminal of the TFT 81 through the TFT 83, the TFT 81 is turned to an OFF state, and the organic EL element 85 is turned to a non-light-emitting state. The TFT 83 is provided for an organic EL display performing time-division gradation driving and area-division gradation driving, in order to carry out writing and erasing of data in parallel. As described above, providing a TFT for data erase for a pixel circuit of an organic EL display separately from a TFT for data write has been conventionally known.